Internal combustion engines are widely used throughout the world for vehicle propulsion, electric power generation, handling of liquids and gases, and in various industrial applications. Fuel and air is combusted within an engine cylinder in a conventional operating scheme to produce a rapid rise in pressure that drives a piston coupled with a crankshaft. Spark-ignited engines typically employ a liquid petroleum distillate fuel such as gasoline, or gaseous fuel such as natural gas, methane, propane, mixtures of these, and various others. Compression-ignition engines utilize fuels such as diesel distillate fuel, biodiesel, and others that can be autoignited with air in a compression stroke of a piston. Research interest in recent years has increasingly gravitated toward flexibility of engines with regard to fuel utilization, especially utilization of gaseous fuels. Fuel prices are often dynamic, and certain gaseous fuels can have combustion or emissions characteristics which it is desirable to exploit. Certain engines allow for operation on both or either liquid fuels such as diesel distillate and natural gas or other gaseous fuels. Diesel alone is relatively easy to autoignite, but can have undesirable emissions. Natural gas, on the other hand, in some instances can exhibit ignition problems such as ignition failure or knock, or suffer from problems of combustion stability. In so-called lean burn applications, where gaseous fuel and air are attempted to be burned at a stoichiometrically lean equivalence ratio, such challenges can be particularly acute.
Dual fuel engines where a combustion-initiating pilot injection of liquid fuel is used to ignite a main charge of gaseous fuel address some of these issues with combustion predictability and controllability. In still other proposed dual fuel engines, a spark plug can be used to ignite the main charge of gaseous fuel, with liquid fuel injection used when operating in a diesel-only mode. Each of these general approaches suffer from a variety of drawbacks, but have certain advantages. Packaging concerns where both a fuel injector and a spark plug are used in the same engine exist, as well as potentially increased costs with an increased number of parts. One example of a dual fuel engine employing a diesel pilot fuel to ignite natural gas is known from U.S. Pat. No. 6,032,617 to Willi, et al.